It is a fundamental concept of systems theory that every system or subsystem is characterized by a frequency response to a stimulating spectrum of signals. Some subsystems, such as measurement probes, include a tunable frequency compensation network. The compensation network of the probes is adjustable for a substantially flat frequency response, so as to provide for accurate measurements, when measurements are made using an instrument, such as an oscilloscope, coupled through the measurement probes to a device under test. In general, failure to make adequate adjustments to the probe compensation may cause undesirable measurement errors.
Various manual ways of adjusting probe compensation are known in the prior art. For example, after a probe is coupled with an oscilloscope input, an operator of the oscilloscope manually verifies that the probe is functioning correctly and is correctly compensated, by manually connecting the probe to a compensation signal source provided on a front panel of the oscilloscope. The oscilloscope provides a display and measurement of the compensation signal, so that the operator can deliberate and determine whether the probe is functioning correctly and is correctly compensated.
While the prior art provides some advantages, some limitations still remain. In particular, since the probe compensation signal source shares a common ground with the oscilloscope input, this method does not verify that a ground lead of the probe is functional, and therefore does not provide sufficient information for the operator to determine whether the probe is functioning correctly. Because of this, the operator must expend additional effort and manually verify that the ground lead is also functional by some other method, such as, for example, measuring the ground lead with an ohmmeter, or shorting the signal at the probe signal lead with the ground lead.
Additionally, it should be noted that on typical oscilloscopes, the compensation signal has a fixed amplitude. Accordingly, the manual method of the prior art has limited flexibility, since the probe is verified for only a limited amount of the fixed amplitude.
Another limitation is that the manual method of the prior art is time consuming, on the order of one to two minutes of time for each probe. Furthermore the manual method of the prior art is subject to errors caused by an inattentive or untrained operator. There is an unfortunate synergy between these two factors. Because of the time required, the operator may be more prone to errors, or may even skip verification of probe functionality and compensation altogether, in order to save time. Paradoxically, skipping probe verification may result in even more time lost, because the operator may make measurements for many minutes before realizing that the probe is not functioning correctly. This is especially true if the probe ground lead is faulty.
What is needed is an accurate, quick, simple, and easy to use method and apparatus for automatic verification of measurement probe functionality and compensation for a wide range of signal amplitude levels.